(1) Field of the Invention
This invention relates to optical isolators for use in optical fiber communication or in optical fiber amplifiers.
(2) Description of the Related Art
A known optical isolator of little polarization dependence is formed of birefringent crystals an Faraday elements. A construction of such an optical isolator will be described with reference to FIG. 1.
Generally, rutile (TiO.sub.2) is used as the birefringent crystals. The optical isolator shown in FIG. 1 includes a rutile plate 11, a Faraday element 21, a rutile plate 12 and a rutile plate 13 arranged in the stated order from light incident end. The rutile plate 11 has a thickness .sqroot.2 t, while the rutile plates 12 and 13 have a thickness t. The rutile plates 11, 12 and 13 have optic axes (C axes) oriented as shown in FIGS. 2A, 2B and 2C when seen from the light incident end, respectively. That is, the rutile plate 12 has an optic axis extending in a direction rotated 45.degree. clockwise from the optic axis of the rutile plate 11. The rutile plate 13 has an optic axis extending in a direction rotated 135.degree. clockwise from the optic axis of the rutile plate 11.
Assume that light enters the first rutile plate 11 at a position P1 shown in FIG. 3A. A component of the incident light having a vertical plane of polarization is refracted as an extraordinary light component, while a component having a horizontal plane of polarization travels straight through as an ordinary light component without being refracted. As a result, the two components of the light entering at position P1 exit at separate positions P1 and P2 on an exit end surface as shown in FIG. 3B. The distance between positions P1 and P2 is proportional to the thickness .sqroot.2 t of the rutile plate 11.
The two polarized light components exiting separately have the respective planes of polarization rotated 45.degree. in the Faraday element 21. Since the positional relationship between the two light components does not change, the two light components entering the rutile plate 12 have planes of polarization and incident positions as shown in FIG. 3C. Of the light components entering the rutile plate 12, the component entering at position P1 is refracted as an extraordinary light component and exits at position P3 as shown in FIG. 3D. The component entering at position P2 is an ordinary light component which passes straight through without being refracted. Position P3 is located 45.degree. upward and rightward from position P1, and the distance therebetween is proportional to the thickness t of the rutile plate 12.
The two light components exiting the rutile plate 12 enters the rutile plate 13. Of the light components entering the rutile plate 13, the component entering at position P2 is refracted as an extraordinary light component and exits at position P3 as shown in FIG. 3E.
Position P3 is located 45.degree. rightward and downward from position P2, and the distance therebetween is proportional to the thickness t of the rutile plate 13. The component entering at position P3 is an ordinary light component which passes straight through the rutile plate 13 without being refracted, and exits at position P3. Consequently, the two polarized light components coincide at exit position P3. FIG. 4 shows the positional relationship between the polarized components of the incident light, and sequence of variations occurring in the relationship until exit from the optical isolator.
Conventionally, the above optical isolator is used in series connection with another one-stage type optical isolator as shown in FIG. 1, in order to increase a loss in the reverse direction.
Aside from the foregoing optical isolator, a two-stage type optical isolator having a relatively simple construction has been proposed by Chang et al (Kok Wai Chang & Wayne V. Sorin, "High-performance single-mode fiber polarization-independent isolators", Opt. Lett. Vol. 15, No. 8, pages 449-451, 1990). This optical isolator will be described with reference to FIG. 5.
This optical isolator includes a rutile plate 14, a Faraday element 22, a rutile plate 15, a Faraday element 23 and a rutile plate 16 arranged in the stated order from a light incident end. The rutile plates 14 and 16 have a thickness t, while the rutile plate 15 has a thickness .sqroot.2 t. The rutile plates 14, 15 and 16 have optic axes oriented as shown in FIGS. 6A, 6B and 6C when seen from the light incident end, respectively. That is, the rutile plate 15 has an optic axis extending in a direction rotated 45.degree. clockwise from the optic axis of the rutile plate 14. The rutile plate 16 has an optic axis extending in a direction rotated 90.degree. clockwise from the optic axis of the rutile plate 14.
Assume that light enters the first rutile plate 14 at a position P1 shown in FIG. 7A. A component of the incident light having a vertical plane of polarization is refracted as an extraordinary light component, while a component having a horizontal plane of polarization travels straight through as an ordinary light component without being refracted. As a result, the two components of the light entering at position P1 exit at separate positions P1 and P2 on an exit end surface as shown in FIG. 7B. The distance between positions P1 and P2 is proportional to the thickness t of the rutile plate 14.
The two polarized light components exiting separately have the respective planes of polarization rotated 45.degree. in the Faraday element 22. Since the positional relationship between the two light components does not change, the two light components entering the rutile plate 15 have planes of polarization and incident positions as shown in FIG. 7C. Of the light components entering the rutile plate 15, the component entering at position P1 is refracted as an extraordinary light component and exits at position P3 as shown in FIG. 7D. The component entering at position P2 is an ordinary light component which passes straight through without being refracted. Position P3 is located 45.degree. upward and rightward from position P1, and the distance therebetween is proportional to the thickness .sqroot.2 t of the rutile plate 15.
The two polarized light components exiting the rutile plate 15 have the respective planes of polarization rotated 45.degree. in the Faraday element 23. Since, as noted above, the positional relationship between the two light components does not change, the two light components entering the rutile plate 16 have planes of polarization and incident positions as shown in FIG. 7E.
Of the light components entering the rutile plate 16 the component entering at position P2 is refracted as an extraordinary light component and exits at position P3 as shown in FIG. 7F. Position P3 is located rightward from position P2, and the distance therebetween is proportional to the thickness t of the rutile plate 16. The component entering at position P3 is an ordinary light component which passes straight through the rutile plate 16 without being refracted, and exits at position P3. Consequently, the two polarized light components coincide at exit position P3. FIG. 8 shows the positional relationship between the polarized components of the incident light, and sequence of variations occurring in the relationship until exit from the optical isolator.
The conventional optical isolators described above have the following disadvantages.
The optical isolator of FIG. 1 has the disadvantage that the two polarized light components travel through different optical path lengths from entry to the isolator to exit therefrom as shown in FIG. 4. This presents no problem if incident light is in the form of parallel rays. However, convergent or divergent light would form different focal points as a result of the different optical path lengths, which increases a coupling loss between the optical isolator and an exit side optical fiber. Further, this optical isolator is connected in series with another isolator of the same type, which results in a complicated construction with an increased number of optical elements constituting the optical isolators.
The optical isolator of FIG. 5 has a relatively simple construction. However, this isolator also has the disadvantage of providing different optical path lengths for the two polarized light components as shown in FIG. 8.